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1DesignAndApplicationGuideForHighSpeedMOSFETGateDriveCircuitsByLaszloBaloghABSTRACTThemainpurposeofthispaperistodemonstrateasystematicapproachtodesignhighperformancegatedrivecircuitsforhighspeedswitchingapplications.Itisaninformativecollectionoftopicsofferinga“one-stop-shopping”tosolvethemostcommondesignchallenges.Thusitshouldbeofinteresttopowerelectronicsengineersatalllevelsofexperience.Themostpopularcircuitsolutionsandtheirperformanceareanalyzed,includingtheeffectofparasiticcomponents,transientandextremeoperatingconditions.ThediscussionbuildsfromsimpletomorecomplexproblemsstartingwithanoverviewofMOSFETtechnologyandswitchingoperation.Designprocedureforgroundreferencedandhighsidegatedrivecircuits,ACcoupledandtransformerisolatedsolutionsaredescribedingreatdetails.AspecialchapterdealswiththegatedriverequirementsoftheMOSFETsinsynchronousrectifierapplications.Several,step-by-stepnumericaldesignexamplescomplementthepaper.INTRODUCTIONMOSFET–isanacronymforMetalOxideSemiconductorFieldEffectTransistoranditisthekeycomponentinhighfrequency,highefficiencyswitchingapplicationsacrosstheelectronicsindustry.Itmightbesurprising,butFETtechnologywasinventedin1930,some20yearsbeforethebipolartransistor.ThefirstsignallevelFETtransistorswerebuiltinthelate1950’swhilepowerMOSFETshavebeenavailablefromthemid70’s.Today,millionsofMOSFETtransistorsareintegratedinmodernelectroniccomponents,frommicroprocessors,through“discrete”powertransistors.ThefocusofthistopicisthegatedriverequirementsofthepowerMOSFETinvariousswitchmodepowerconversionapplications.MOSFETTECHNOLOGYThebipolarandtheMOSFETtransistorsexploitthesameoperatingprinciple.Fundamentally,bothtypeoftransistorsarechargecontrolleddeviceswhichmeansthattheiroutputcurrentisproportionaltothechargeestablishedinthesemiconductorbythecontrolelectrode.Whenthesedevicesareusedasswitches,bothmustbedrivenfromalowimpedancesourcecapableofsourcingandsinkingsufficientcurrenttoprovideforfastinsertionandextractionofthecontrollingcharge.Fromthispointofview,theMOSFETshavetobedrivenjustas“hard”duringturn-onandturn-offasabipolartransistortoachievecomparableswitchingspeeds.Theoretically,theswitchingspeedsofthebipolarandMOSFETdevicesareclosetoidentical,determinedbythetimerequiredforthechargecarrierstotravelacrossthesemiconductorregion.Typicalvaluesinpowerdevicesareapproximately20to200picosecondsdependingonthesizeofthedevice.ThepopularityandproliferationofMOSFETtechnologyfordigitalandpowerapplicationsisdrivenbytwooftheirmajoradvantagesoverthebipolarjunctiontransistors.OneofthesebenefitsistheeaseofuseoftheMOSFETdevicesinhighfrequencyswitchingapplications.TheMOSFETtransistorsaresimplertodrivebecausetheircontrolelectrodeisisolatedfromthecurrentconductingsilicon,thereforeacontinuousONcurrentisnotrequired.OncetheMOSFETtransistorsareturned-on,theirdrivecurrentispracticallyzero.Also,thecontrollingchargeandaccordinglythestoragetimeintheMOSFETtransistorsisgreatlyreduced.Thisbasically2eliminatesthedesigntrade-offbetweenonstatevoltagedrop–whichisinverselyproportionaltoexcesscontrolcharge–andturn-offtime.Asaresult,MOSFETtechnologypromisestousemuchsimplerandmoreefficientdrivecircuitswithsignificanteconomicbenefitscomparedtobipolardevices.Furthermore,itisimportanttohighlightespeciallyforpowerapplications,thatMOSFETshavearesistivenature.ThevoltagedropacrossthedrainsourceterminalsofaMOSFETisalinearfunctionofthecurrentflowinginthesemiconductor.ThislinearrelationshipischaracterizedbytheRDS(on)oftheMOSFETandknownastheon-resistance.On-resistanceisconstantforagivengate-to-sourcevoltageandtemperatureofthedevice.Asopposedtothe-2.2mV/°Ctemperaturecoefficientofap-njunction,theMOSFETsexhibitapositivetemperaturecoefficientofapproximately0.7%/°Cto1%/°C.ThispositivetemperaturecoefficientoftheMOSFETmakesitanidealcandidateforparalleloperationinhigherpowerapplicationswhereusingasingledevicewouldnotbepracticalorpossible.DuetothepositiveTCofthechannelresistance,parallelconnectedMOSFETstendtosharethecurrentevenlyamongthemselves.ThiscurrentsharingworksautomaticallyinMOSFETssincethepositiveTCactsasaslownegativefeedbacksystem.Thedevicecarryingahighercurrentwillheatupmore–don’tforgetthatthedraintosourcevoltagesareequal–andthehighertemperaturewillincreaseitsRDS(on)value.Theincreasingresistancewillcausethecurrenttodecrease,thereforethetemperaturetodrop.Eventually,anequilibriumisreachedwheretheparallelconnecteddevicescarrysimilarcurrentlevels.InitialtoleranceinRDS(on)valuesanddifferentjunctiontoambientthermalresistancescancausesignificant–upto30%–errorincurrentdistribution.DevicetypesAlmostallmanufacturershavegottheiruniquetwistonhowtomanufacturethebestpowerMOSFETs,butallofthesedevicesonthemarketcanbecategorizedintothreebasicdevicetypes.TheseareillustratedinFigure1.n+n+n+Substraten-EPIlayerGATESOURCEDRAINppn+n+n+Substraten-EPIlayerGATESOURCEDRAINp+p+(a)(b)n+n+SubstratepGATESOURCEDRAINpn(c)OXIDEFigure1.PowerMOSFETdevicetypesDouble-diffusedMOStransistorswereintroducedinthe1970’sforpowerapplicationsandevolvedcontinuouslyduringtheyears.Usingpolycrystalline